Soldering with resilient contacts

ABSTRACT

A method of making a solder connection. An element bearing a solder mass is forcibly engaged with another element bearing a resilient metallic contact so that the contact wipes the surface of the solder mass and so that the contact is deformed and bears against the wiped surface. While the contact is in its deformed condition, the contact and solder mass are brought to an elevated bonding temperature sufficient to soften the solder, so that the contact penetrates into the solder mass under the influence of its own resilience. The contact bonds with the pure solder inside the solder mass, so that the effective bonding can be achieved even without flux.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation in part of U.S. patentapplication No. 08/306,205, filed Sep. 14, 1994, pending, which in turnis a continuation in part of U.S. patent application No. 08/254,991filed Jun. 7, 1994, pending. The disclosures of both of saidapplications are hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

The present invention relates to soldered connections formicroelectronic devices such as semiconductor chips and the associatedcircuit panels, and to methods of making and using such connections.

Microelectronic circuits require numerous connections between elements.For example, a semiconductor chip may be connected to a small circuitpanel or substrate, whereas the substrate may in turn be connected to alarger circuit panel. The chip to substrate or "first level"interconnection requires a large number of individual electrical inputand output ("IO") as well as power and ground connections. As chips havebecome progressively more complex, the number of I/O connections perchip has grown so that hundreds of connections or more may be needed fora single chip. To provide a compact assembly, all of these connectionsmust be made within a relatively small area, desirably an area about thearea of the chip itself. Thus, the connections must be densely packed,preferably in an array of contacts on a regular grid, commonly referredto as a "Bump Grid Array" or "BGA". The preferred center-to-centerdistance between contacts or "contact pitch" for chip mountings is onthe order of 1.5 mm or less, and in some cases as small as 0.5 mm. Thesecontact pitches are expected to decrease further. Likewise, chipmounting substrates and other circuit panels used in microelectronicshave become progressively more miniaturized, with progressively greaternumbers of electrical conductors per unit area. Connectors for theseminiaturized panel structures desirably also have very small contactpitch. Connections of chip mounting substrates to other elements arereferred to as "second-level" interconnections.

Microelectronic connections must meet numerous, often conflictingrequirements. As mentioned above, the size of the device poses a majorconcern. Further, such connections often are subject to thermal cyclingstrains as temperatures within the assembly change. The electrical powerdissipated within a chip or other microelectronic element tends to heatthe elements so that the temperatures of the mating elements rise andfall each time the device is turned on and off. As the temperatureschange, the various connected elements expand and contract by differentamounts, tending to move the contacts on one element relative to themating contacts on the other element. Changes in the temperature of thesurrounding environment can cause similar effects which producemechanical stress in the connected components.

The connections must also accommodate manufacturing tolerances in thecontacts themselves and in the connected elements. Such tolerances maycause varying degrees of misalignment. Additionally, contamination onthe surfaces of the mating contact parts can interfere with theconnection. Therefore, the contact system should be arranged tocounteract the effects of such contaminants. For example, in makingsoldered connections, oxides and other contaminants must be removed byfluxes. These fluxes in turn can contaminate the finished product.Although these fluxes can be removed by additional cleaning steps, orcan be formulated to minimize ill-effects on the finished product, itwould be desirable to provide soldered connections which minimize oreliminate the need for Such fluxes. All of these requirements, takentogether, present a formidable engineering challenge.

Various approaches have been adopted towards meeting these challenges.

Certain preferred embodiments disclosed in our aforementioned U.S.patent application No. 08/254,991 provide connectors for mounting amicroelectronic element such as a semi-conductor chip or other element.Connectors according to these embodiments include a planar dielectricbody having first and second surfaces and also having a plurality ofholes open to the first surface. The holes are disposed in an arraycorresponding to an array of bump leads on the device to be mounted. Theconnector further includes an array of resilient contacts secured to thefirst surface of the dielectric body in registration with the holes sothat each such contact extends over one hole. Each contact is adapted toresiliently engage a bump lead inserted into the associated hole. A chipor other microelectronic component with the bump leads thereon can beconnected to the contacts by superposing the microelectronic element onthe dielectric body of the connector so that the microelectronic elementoverlies the first surface and so that the bump leads on the elementprotrude into the holes and are engaged by the resilient contacts.Preferred connector components according to this aspect of the inventionwill establish electrical connection with the bump leads by mechanicalinter-engagement of the bump leads and contacts.

Each contact may include a structure such as a ring of a sheet-likemetallic contact material overlying the first surface of the dielectricbody and fully or partially encircling the opening of the associatedhole, and each contact may also include one or more projections or tabsformed integrally with the ring and extending inwardly therefrom overthe hole. Preferably, a plurality of such projections are provided atcircumferentially-spaced locations around the hole. These projectionsare arranged so that when a bump lead enters the hole, it tends to forcethe projections downwardly and outwardly, away from one another. Theprojections tend to center the bump in the hole. The chip or othermicroelectronic component can be reliably connected simply by pressingthe chip against-the connector in proper alignment with the holes. Thisreliable interconnection can be used either as a temporaryinterconnection for testing purposes or as a permanent connection.

As set forth in the No. '991 application, the motion of the bump leadsentering the holes as the microelectronic element is engaged with theconnector causes the bump leads to wipe across the contacts so as toclean debris, oxides and other contaminants from the surfaces of thecontact and bump lead. The bump leads may include a bonding materialsuch as a solder, to form a permanent metallurgical connection with thecontacts. Thus, the microelectronic component can be engaged with theconnector and tested using the mechanically-made electricalinterconnections. If the results are satisfactory, the permanentmetallurgical bond can be formed by heating to melt the solder.

Certain aspects of our U.S. patent application No. 08/306,205 providecontacts for a microelectronic device, which contacts can be used in theconnectors of the No. '991 application and in other structures.According to these aspects of the No. '205 application, each contactincludes a base portion defining a base surface, and one or moreasperities preferably integral with the base portion and protrudingupwardly from the base surface. Each such asperity desirably defines atip remote from the base surface and a substantially sharp feature atthe tip. The base portion of each contact may include one or moremetallic layers such as copper or copper-bearing alloys, and may alsoinclude a polymeric structural layer in addition to a conductive,desirably metallic, layer.

The base portion of each contact may include an anchor region and atleast one tab or projection formed integrally with the anchor region.The asperity or asperities may be disposed on each tab at a distal end,remote from the anchor region. In use, the anchor region of such acontact is fixed to a connector body or other support, whereas the tabis free to bend. When a bump lead is engaged with the tab, the tab bendsand the mating bump lead and tab move relative to one another to providea wiping motion. The resilience of the tab causes the sharp feature ofthe asperity to bear on the mating element and scrape the matingelement. The scraping action promotes reliable contact before bonding,as well as reliable bonding. The anchor region of each contact may bepart of a substantially ring-like common anchor region. A contact unitmay include such common anchor region and a plurality of tabs extendinginwardly from the ring-like anchor region towards a common center.

SUMMARY OF THE INVENTION

One aspect of the present invention provides methods of making anelectrical connection. A method according to this aspect of theinvention preferably includes the step of forcibly engaging a firstelement bearing one or more masses of an electrically conductive fusiblebonding material such as a solder or other fusible conductivecomposition and a second element bearing one or more resilient,electrically conductive contacts so that the contact wipes the surfaceof the mass and so that the contact is deformed and bears against thewiped surface. The method further includes the step of bringing thecontact and the mass to an elevated bonding temperature sufficient tosoften the fusible bonding material, so that the contact penetrates intothe mass under the influence of its own resilience and then cooling theengaged contact and mass. The heating step typically is performed afterthe engaging step, during the engaging step the solder mass is cool andsolid. Most preferably, the engaging step is performed so that thecontacts wipe the surfaces of the masses during the engagement step.Most preferably, each contact has one or more asperities on its surface.Typically, the heating and cooling steps are performed by heating andcooling the entire assembly, including both elements.

When the fusible material is heated and softened, and the contactspenetrate into the masses, each contact is exposed to substantially purefusible material. For example, where the fusible material is a solder,each contact is exposed to substantially pure solder, free of oxides andother impurities found at the solder mass surfaces. This facilitatesformation of a sound, metallurgical bond between the solder and thecontact. Fluxes need not be utilized to remove the impurities from thesurfaces of the solder masses. The contacts may incorporateoxidation-resistant materials at the regions which penetrate into thesolder during the process. This further facilitates formation of thesolder joint without the use of fluxes.

Although the present invention is not limited by any theory ofoperation, it is believed that the wiping action tends to rupture thefilm or layer of oxide which may be present on the surfaces of thesolder masses, and thus facilitates penetration of the contacts into theunderlying pure solder during the subsequent heating and softeningsteps. These contacts desirably are provided with sharp-featuredasperities in the region which scrape the surfaces of the solder masses.

Further aspects of the present invention include bonded articles. Anarticle according to this aspect of the invention includes a firstelement having a structure with at least one terminal thereon, masses ofa bonding material such as a solder and a second element having a bodywith one or more contacts thereon, each such contact including an anchorportion secured to the body and at least one tab projecting fromthe-anchor portion and having a distal end remote therefrom. The distalend of each such tab projects into one mass of bonding material on thefirst element and is bonded thereto. Preferably, the bonding material isa solder and the distal end of each tab is metallurgically bonded to themass. Each tab desirably is bonded to a portion of the mass remote fromthe surface of the mass. Most preferably, each contact incorporates anannular anchor region defining a central axis and a plurality of tabsprojecting inwardly towards such central axis. Each mass is receivedwithin the annular anchor portion of one such contact and is penetratedby the radially inwardly extending tabs of such contact. Desirably, tabspenetrate into each mass into many directions, so that the tabssubstantially surround the mass. As further discussed below, connectionsaccording to this aspect of the invention provide particularly stronginterconnections, even where the individual tabs are quite small andhence quite fragile.

These and other objects, features and advantages of the presentinvention will be more readily apparent from the detailed description ofthe preferred embodiments set forth below, taken in conjunction with theaccompany drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary, diagrammatic perspective view depictingportions of a connector used in one embodiment of the present invention.

FIG. 2 is a fragmentary, diagrammatic partially sectional view on anenlarged scale along lines 2--2 in FIG. 1.

FIG. 3 is a view similar to FIG. 2 but illustrating the connector duringone state of a process in accordance with one embodiment of theinvention.

FIGS. 4 is a view similar to FIG. 3 but depicting a later stage of theprocess.

FIG. 5 is a fragmentary elevational view depicting components used in afurther embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A connector useful in one embodiment of the invention may besubstantially in accordance with certain embodiments of theaforementioned co-pending applications. The connectors include aplurality of independent, electrical contact units 29. Each contact unitincludes four contacts 20. Each contact 20 includes a small metallic tabincorporating a base layer 22 (FIG. 2) defining an upwardly facing basesurface 24. The base portion of each contact desirably is formed from aresilient metal selected from the group consisting of copper,copper-bearing alloys, stainless steel and nickel. Beryllium copper isparticularly preferred. The base portion desirably may be between about10 and about 25 microns thick. A layer 25 of an etch-resistant metalsuch as nickel or gold used in the contact formation process may bedisposed on base surface 24. Layer 25 desirably is between about 0.5 and2.0 microns thick. Each such tab is joined to a generally square,ring-like anchor portion 26 integral with the tab. Each tab has a tip 28at the distal end of the tab remote from the anchor portion.

Four tabs extend inwardly from each anchor portion 26, the tabs beingseparated from one another by channels 23. Each contact or tab 20 has anasperity 30 projecting upwardly from the base surface 24 adjacent thetip 28 or distal end of the tab, remote from the anchor portion. Eachasperity includes a column 32 of a first or base metal integral withbase portion 22 and further includes a cap 34 overlying the column 32 atthe uppermost tip of the asperity, remote from base surface 24. Eachcolumn 32 is generally cylindrical or frustoconical in shape, so thatthe tip of each column is substantially circular. The cap of each columndefines a flat, circular tip surface and substantially sharp edge 36encircling the tip surface. Each asperity desirably protrudes upwardlyfrom the base surface less than about 100 microns, more preferablybetween about 5 microns and about 40 microns, and most preferablybetween about 12 microns and about 25 microns. Each asperity may bebetween about 12 and about 75 microns in diameter, more preferably about12 to about 35 microns in diameter. The cap metal 34 may be selectedfrom the group consisting of metals resistant to etching by etchantswhich etch the first or base metal. Cap metals selected from the groupconsisting of gold, silver, platinum, palladium, osmium, rhenium,nickel, tin and combinations thereof are preferred. As discussed indetail in the aforementioned No. '205 application, such etch-resistantmetals aid in formation of sharp edges 36. Thus, in formation of thecontact, the etch-resistant metal may act as an etch mask during etchingof the base layer to form columns 32. Moreover, the harderetch-resistant metals, particularly nickel, osmium and rhenium, aid inpreserving the edge during use. The contact surface which will engagethe solder mass in use includes the surface of the base portion adjacentthe contact tip and also includes the cap metal. Desirably, the contactsurface includes one or more metals solderable metals selected from thegroup including tin copper, silver, lead, palladium, gold and alloys andcombinations thereof. For example, the contact surface may includealloys such as copper-silver; lead-tin, and gold-tin.

The contact units are disposed on the top surface 38 of a connector body40, and spaced apart from one another so that there are slots 42 betweenadjacent connector units. Connector body 40 incorporates a sheet-likestructural support layer 44 having holes 46 therein. Support layer 44can be formed from a metal such as copper or from a dielectric materialsuch as fiber reinforced polymers, or unreinforced polymers. Thestructural support is covered by a bottom dielectric layer 48 and a topdielectric layer 50, which merge with one another within holes 46, sothat the dielectric layer cooperatively lines the holes as well. Aconductive metallic via liner 52 extends through each hole 46 from thetop surface 38 of the connector body to the opposite, bottom surface 54.Each via liner 52 flares radially outwardly, away from the central axis56 of the associated hole at the bottom surface so as to form an annularterminal 58 at such bottom surface. Each via liner also flaresoutwardly, away from the central axis at the top surface 38 so as toform a contact support structure 60. The periphery of each contactsupport is generally square.

One contact unit 29 is disposed on each contact support structure 60,substantially in alignment with the square boundary thereof. Eachcontact unit is secured to the associated contact support by four posts66 formed integrally with the contact support 60 and extending upwardlythrough holes 64 in the contact unit. Each post 66 has a bulbous portion68 at the end of the post remote from the contact support 60, overlyingbase surface 24. These posts-and bulbous portions thus secure eachcontact unit 29 to the corresponding contact support 60 so that theindividual contacts or tabs 20, and particularly the tips 28 thereof,protrude radially inwardly, toward the axis 56 of the associated hole 46in the connector body and so that .tips of the contacts or tabs 20overly the hole 46. The posts and the contact supports 60 alsoelectrically connect each contact unit to the associated via liner andthus to the terminal 58 on the bottom surface.

Contact units 29, and hence the individual contacts or tabs 20 aredisposed in a regular pattern corresponding to the patterns of holes 46in body 40. The asperities 30 on the contacts are also disposed in aregular pattern, in registration with the pattern of contacts 20, sothat the same number of asperities are disposed on each contact. In theembodiment of FIGS. 1 and 2, only one asperity is disposed on eachcontact. However, because both the asperities and the contacts aredisposed in regular patterns, all of the contacts are provided withasperities. Also, the asperity on each contact is at the same locationnamely, adjacent the tip of the tab or contact, remote from the anchorregion of the contact unit. A layer of a conventional photographicallypatternable solder mask material 67, shown in broken-line phantom viewin FIG. 2 for clarity of illustration, covers the top surfaces of theconnector body and contact unit. Solder mask layer 67 is provided withholes aligned with the holes 46 in the connector body and with thecentral axes 56 of the holes and contact units. The tips or distal ends28 of the contact tabs are exposed within the holes 65 of the soldermask layer, whereas the anchor portions of the contact units, and theproximal ends of the tabs adjacent the anchor portions are covered bythe solder mask layer. The solder mask layer may be formed fromphotoimagable polymers or from other polymers such as epoxies andpolyimide. Thermoplastic materials such as polyetherimide (availableunder the registered trademark Ultem) and fluorocarbons, particularlyfluorinated ethylene-propylene ("FEP") may also be used.

The connector of FIGS. 1 and 2 may be engaged with a larger substrate,such as a multilayer substrate 68 having leads 69. The terminals 58 ofthe connector, and thus the contact units 29, may be electricallyconnected to the internal leads 69 of the substrate by conventionallamination and/or solder bonding methods, or by the lamination andinterconnection methods taught in U.S. Pat. No. 5,282,312, thedisclosure which is hereby incorporated by reference herein.

After assembly to the substrate, the connector of FIGS. 1 and 2 isengaged with a mating microelectronic element 70. Microelectronicelement 70 may be an active microelectronic device such as asemiconductor chip, or may be a circuit-bearing substrate or panel orother device. The microelectronic element has a body having a rearsurface 75 with terminal 73 thereon connected to other electroniccircuitry or components of the microelectronic element (not shown).Terminals 73 are disposed in an array matching the array of contactunits 29 and holes 46 on the connector. A mass of solder 72 is disposedon each terminal 73. The solder masses are in the form of bumpsprotruding upwardly from surface 75. The solder masses desirably includeone or more metals selected from the group consisting of lead, tin,silver, indium and bismuth. Lead-tin alloys; lead-tin-silver alloys; orindium-bismuth alloys may be used.

In a process according to one embodiment of the invention, the rearsurface 75 of microelectronic element 70 is placed adjacent the frontsurface 38 of the connector body 40.

The terminal 73 and solder mass 72 on the microelectronic element arealigned with the hole 46 in the connector body and hence with thecentral axes 56 of the contact units and holes. During this stage of theprocess, the solder mass are at a temperature substantially below themelting temperature of the solder and therefore are in a substantiallysolid, rigid condition. The surface of each solder-mass bears on thetabs of the associated contact unit adjacent the tip or distal end ofthe tab. At least during some portions of this engagement step, thesolder units engage each tab by means of the asperity 30 on the tab. Themicroelectronic element is then forcibly engaged with the connector byforcing the microelectronic element toward the top surface of theconnector body. As illustrated in FIG. 3, the anchor portion orperiphery 26 of each contact unit remains substantially in fixedposition, whereas the distal regions of each tab 20, adjacent the tips28 of the tabs, bend downwardly, in the direction of motion of theengaged solder mass 72. In this condition, a part of each sharp edge 36faces upwardly, in the direction opposite to the downward motion ofmicroelectronic element 70 and solder masses 72. The tip 28 of each tab,and the asperity on such tip are biased inwardly, towards the centralaxis 56 of the hole, by the resilience of the tab 20. The upwardlyfacing portion of each edge 36 tends to dig into the surface of a soldermass 72. The sharp-edged asperity on each tab thus scrapes a path alongthe surface of the solder mass 72.

This scraping action effectively removes oxides and other contaminantsfrom the scraped paths. This assures reliable electrical contact betweencontacts 20 and the solder masses 72. In particular, the tips of theasperities aid in making contact with the solder masses 72. Because thecap metal in layer 34 on the tip of each asperity is a substantiallyoxidation resistant metal, it normally does not have any substantialoxide or contaminant layer. Thus, the solder masses and contacts form afirm, reliable electrical interconnection. This action is repeated ateach contact unit and with each solder mass 72 on the surface of themicroelectronic element, so that reliable interconnections are formedsimultaneously between all of the solder masses 72 and all of theinternal conductors 69 of substrate 68. The electrical connectionachieved by mechanical interengagement of the element may be used as atest connection, so that microelectronic element 70, its connections tosubstrate 68 and the other elements connected to the same substrate canbe tested and operated under power. This test also serves to testelectrical continuity between the solder masses 72 and contacts 20. If adefective connection or component is identified during the test, thesame can be removed and replaced readily. Ordinarily, the connector andcontacts can be reused.

Following completion of the testing step, the engaged solder masses andcontacts are heated to an elevated bonding temperature sufficient tosoften the solder. Preferably, this heating is accomplished withoutdisengaging the solder masses from the contacts. The solder massesremain in engagement with the contacts from completion of the testingstep throughout the remainder of the process. The bonding temperature issufficient to soften the solder, as by converting some or all of thesolder to a liquid phase. The solder masses and contacts may be heatedby exposing the entire assembly to any suitable heating medium, such asa hot gas, radiant energy or the like. As the solder softens, each tab20 springs back from the deformed condition of FIG. 3 towards itsoriginal, undeformed shape. As shown in FIG. 4, the distal ends or tipsof the tabs penetrate into the solder mass. Although the presentinvention is not limited by any theory of operation, it is believed thatthe wiping action between the contacts and the surface of the soldermass during the engagement step, and particularly, the scraping actionof the asperities on the contacts facilitates penetration of the soldermass surface by the contacts. It is believed that disruption of theoxide layer during the wiping and scraping action materially reduces thecoherence of the oxide layer after softening of the solder masses. Thisin turn allows the contact tips to break through the oxide layer readilyand to enter into the substantially pure, unoxidized solder beneath theoxide layer. The wiping action also tends to remove oxides or othercontaminants from the surfaces of the contact tips.

Because the tips of the contacts are immersed in the pure, unoxidizedsolder, they are readily wetted by the solder. Provided that at leastsome portions of the contact tips are substantially free of oxidationalor other impurities at this stage of the process, the pure solder caneffectively wet the contact tips. No fluxes or other extraneous chemicalagents are required. The assembly desirably is held at the elevatedbonding temperature for a period sufficient to ensure that all parts ofthe assembly reach the bonding temperature. The contacts tend to recoverand penetrate into the solder masses to substantially instantaneouslyafter the solder masses soften. When the solder wets the contact tips,it tends to flow along the surfaces of the contacts under the influenceof capillary action. This tends to draw the solder outwardly, away fromthe tips of the contacts and towards the anchor regions of the contacts.The solder mask layer 67 blocks such outward flow. Where the solder masklayer includes a material such as a thermoplastic material or othermaterial which can be softened by heat, the solder mask may be broughtto a flowable condition when the assembly is heated to the bondingtemperature. The solder mask layer thus flows into conforming contactwith the surface 75 of the microelectronic element and bonds to thesurface. This forms a seal which encloses each contact and solder massin an individual cavity, thus protecting the connections from theenvironment. Alternatively or additionally, the solder mask layer mayinclude an adhesive, such as an acrylate adhesive, which bonds tosurface 75.

After the assembly has been heated to the bonding temperature, it iscooled to a temperature below the bonding temperature, typically to roomtemperature, so as to resolidify the solder. The resulting assemblyincludes solid solder masses 72 penetrated by the contact tabs 20 andmetallurgically bonded to the contact tabs. Each solder mass protrudesgenerally in an axial or Z direction, generally perpendicular to the topsurface 38 of the connector body and generally parallel to the axis 56of the associated hole 46 in the connector body. Thus, each solder massextends axially through the center of one ring-like anchor region 26.The contact tabs associated with such tabs associated with suchring-like anchor region extend generally radially inwardly into thesolder mass. This configuration provides a particularly strong bond.Most preferably, the array of contact tabs penetrating into each soldermass substantially surrounds the solder mass in all radial directions.Thus, movement of the solder masses relative to the contacts in anyradial or X-Y direction tends to force the contact into even moreintimate engagement with one or more of the contact tabs. Although thecontact tabs are illustrated in FIG. 4 as being fully restored to theoriginal, undeformed condition, this is not essential. Thus, the contacttabs, after the softening and cooling steps, may have some residualdeformation in the axial direction, into the hole, so that each contacttab slopes both axially and radially.

Other contact configurations may be employed. For example, asillustrated in FIG. 5, a microelectronic element or first element 170having terminals 173 on its rear surface 175 and solder masses 172 onthe terminals may be engaged with a connector or second element 140having contacts 122 in the form of elongated strips on the front surface138 of the element body. Each contact 122 has an anchor portion 126connected to the internal electrical element or circuitry (not shown) ofsecond element 140 and also has a cantilevered elongated tab 120 havinga sharp asperity 130 at the distal end or tip 128 of the tab remote fromthe anchor portion 126. In its undeformed condition, each tab extendsoblique to surface 138. When the first element 170 is forced towards thesecond element 140, solder masses 172 bend the tabs downwardly to thedeformed position shown at 120' in broken lines in FIG. 5. This actioncauses the tips of the tabs, and particularly the asperities 130, tomove in horizontal directions generally parallel to the surface 138 ofthe second element and generally transverse to the motion of the soldermasses and first element 170, thus causing the tips of the tabs andparticularly the aperities, to wipe the surfaces of the solder masses.Here again, the solder masses are heated, as by heating both elements.The contact tabs 120 spring back at least partially towards theirundeformed positions and thus penetrate into the solder masses. Onceagain, after the assembly is cooled and the solder masses resolidified,each solder mass has a contact tab penetrating into it andmetallurgically bonded to it.

The contacts described above can be adapted to provide various levels ofinterengagement force between each contact and the engaged matingelectrical element. Interengagement forces between about 0.5 and about 5grams force per engaged contact or tab are preferred in typicalmicroelectronic applications. The total interengagement force percontact unit, and hence the total interengagement force per solder mass,desirably is between about 2 and about 20 grams force. With thesharp-edged asperity structures discussed above, these relatively smallinterengagement forces nonetheless provide effective scraping and wipingaction. The ability to provide effective scraping action at relativelylow force levels is especially significant where numerous contacts mustbe engaged with numerous solder masses. The degree of wipe or relativemovement between the asperity edge and the mating surface during contactengagement can be relatively small, typically less than about 20 micronsand usually between about 5 and 10 microns. Even this small relativemovement however is enough for the sharp features of the asperity tipsto break through the contaminants on the surface of the solder masses.

In the embodiments discussed above, the bonding material in the masses72 is a solder. Other softenable bonding materials can be employed, suchas polymeric materials filled with conductive particles. For example,partially-cured or "B-staged" epoxy resins, polyimide-siloxane resins orthermoplastic polymers filled with metallic particles may be employed.The metallic particles may be formed from metals such as silver, gold,palladium and combinations and alloys thereof, such as silver-palladium,silver alloys and gold alloys.

As these and other variations and combinations of the features set forthabove can be utilized without departing from the invention, theforegoing description of the preferred embodiments should be taken byway of illustration rather than by way of limitation of the invention asdefined by the claims.

What is claimed is:
 1. A method of making an electrical connectioncomprising the steps of:(a) forcibly engaging a first element bearing amass of a bonding material and a second element bearing a resilientmetallic contact so that said contact wipes the surface of said bondingmaterial mass and so that said contact is deformed and bears against thewiped surface; and (b) bringing the contact and the bonding materialmass to an elevated bonding temperature sufficient to soften the bondingmaterial, so that the contact penetrates into the solder mass under theinfluence of its own resilience; and (c) cooling the engaged contact andmass.
 2. A method as claimed in claim 1 wherein said step of bringingthe contact and the mass to said elevated bonding temperature isperformed by heating the contact and mass after said engaging step.
 3. Amethod as claimed in claim 2 further comprising the step of testing theengaged mass and contact for electrical continuity after said engagingstep but before said heating step.
 4. A method as claimed in claim 2wherein said first element bears a plurality of solder masses and saidsecond element bears a plurality of resilient metallic contacts, saidengaging step being performed so as to engage said plural contacts andsolder masses with one another simultaneously.
 5. A method as claimed inclaim 4 wherein said second element includes a connector body and eachsaid contact includes an anchor region connected to said body and one ormore cantilevered tabs projecting from said anchor region, said tabsbeing bent during said engaging step and springing back at leastpartially from the bent position as the contacts penetrate into themasses.
 6. A method as claimed in claim 5 wherein at least some of saidtabs have asperities thereon, said asperities scratching the surfaces ofsaid masses during said engagement step.
 7. A method as claimed in claim5 wherein said connector body has a front surface and a plurality ofholes opening through said front surface, said tabs projecting at leastpartially across said holes, said first element including a structurehaving a rear surface, said masses projecting outwardly from said rearsurface, said engaging step including the step of juxtaposing said rearsurface of said structure and said front surface of said connector bodyso that said masses are engaged in said holes.
 8. A method as claimed inclaim 1 or claim 3 or claim 4 or claim 7 wherein said bonding materialin said masses includes a solder.
 9. A method as claimed in claim 8wherein said steps are performed in the absence of flux.
 10. A method asclaimed in claim 8 wherein said contacts include one or more metalsselected from the group consisting of tin, copper, silver, lead,palladium, gold and alloys and combinations thereof, said one or moremetals being present at surfaces of the contacts which engage saidmasses.
 11. A method as claimed in claim 8 wherein said solder includesone or more metals selected from the group consisting of lead, tin,silver, indium and bismuth.
 12. A method as claimed in claim 1 or claim3 or claim 4 or claim 7 wherein said bonding material in said massesincludes a polymeric material.
 13. A method as claimed in claim 4wherein said anchor region of each said contact is covered by a soldermask, said solder in each said mass not wetting said solder mask wherebysaid solder mask limits wicking of solder along said tabs.
 14. A methodas claimed in claim 13 wherein said solder mask forms a bond betweensaid first element and said second element.
 15. A method as claimed inclaim 14 wherein said solder mask includes a thermoplastic dielectricmaterial, said heating step being performed so as to bring saidthermoplastic dielectric material to a flowable condition to therebyform said bond.